1. Field of the Invention
The present invention relates to a horizontal contour correction circuit for applying horizontal contour correction of an image to a video signal for a television display device or the like.
2. Description of the Related Art
Video display devices for displaying analogue video signals on CRTs by means of electronic beam scanning and those for displaying digitized video signals on liquid crystal or plasma displays having fixed pixels are available. Such a video display device comprises a circuit for applying horizontal contour correction for adjusting image sharpness.
FIG. 9 shows a horizontal contour correction circuit 100 for correcting a horizontal contour of a digitized video signal being input in a time serial manner. The shown horizontal contour correction circuit 100 is the one having the simplest structure among general horizontal contour correction circuits, specifically having two serially connected memories 10a and 10b and an operation circuit 12.
The memories 10a and 10b are serially connected, and a video signal to be subjected to horizontal correction is input into an input terminal Tia in a time serial manner. Every receipt of a reset signal, the memories 10a and 10b capture, and store therein, pixel data via the respective input terminals Ti and output pixel data held therein via the respective output terminals To.
Video signals being time serially input via the input terminal Tia of the memory 10a are sequentially transferred for every pixel from the output terminal Toa of the memory 10a to the output terminal Tob of the memory 10b. Provided that a value output from the output terminal Toa is a reference value at a reference time, a value output from the input terminal Tia is of pixel data which is input after the reference time by an amount of time corresponding to a single pixel, and a value output from the output terminal Tob is pixel data which is input before the reference time by an amount of time corresponding to a single pixel.
An operation circuit 12 receives outputs from the input terminal Tia and output terminal Toa of the memory 10a and also from the output terminal Tob of the memory 10b, and executes an operation using the received outputs. Specifically, values output from the input terminal Tia and output terminal Tob are subtracted from an output value from the output terminal Toa, which is assumed as a reference value, and the operation result is output as a result of horizontal contour correction.
FIG. 10 shows a frequency characteristic to be exhibited as a result of horizontal contour correction using the horizontal contour correction circuit 100. In the drawing, the abscissa indicates normalized frequency, while the ordinate indicates an attenuation amount according to a frequency.
FIG. 11 shows a horizontal contour correction circuit 102. The shown horizontal contour correction circuit 102 has an identical structure to that of the circuit of FIG. 9, except that it has four serially connected memories 10a to 10d. 
The operation circuit 12 receives values output from the input terminal Tia of the memory 10a, the output terminal Tob of the memory 10 b, and the output terminal Tod of the memory 10d. The operation circuit 12 subtracts values of the input terminal Tia and the output terminal Tod from a value of the output terminal Tob of the memory 10b, or a reference value, and outputs the result of the operation. FIG. 12 shows frequency characteristic of the horizontal contour correction circuit 102.
As described above, an operation using desirably selected pixel data which is available before and after a reference time, enables formation of horizontal contour correction circuits having a variety of frequency characteristics.
Moreover, when selection circuits 14a and 14b, including a switch, are provided, as shown in FIG. 13, so as to desirably switch, and output, pixel data inputted thereto, a horizontal contour correction circuit capable of selecting different frequency characteristics can be formed.
Further, when a selection control circuit 16 is additionally provided to output a judgment signal S to the selection circuits 14a and 14b based on the absolute value and/or variation of a reference value, outputs from the selection circuits 14a and 14b can be automatically switched based on the judgment signal S. This arrangement enables automatic shift to an appropriate frequency characteristic.
Despite the features described above, in an attempt to further increase the number of frequency characteristics available, additional memories 20e to 20f are required, as shown in the horizontal contour correction circuit 200 of FIG. 14, so that data to use in an operation can be selected from among a plurality of pixel data. Accordingly, the selection circuits 24a and 24b are required to be modified so as to have a multi-contact switch.
Further, in order to achieve automatic selection using the selection control circuit 26, a complicated determination algorithm is required as one item of pixel data must be selected from among a plurality of pixel data and a judgment signal S of many values must be output.
Still further, a mechanism for switching the multi-contact switch of the selection circuits 24a and 24b based on the judgment signal S is also required. This results in an expensive control circuit having a complicated structure.